Semiconductor devices are commonly found in modern electronic products. Semiconductor devices vary in the number and density of electrical components. Semiconductor devices perform a wide range of functions such as analog and digital signal processing, sensors, transmitting and receiving electromagnetic signals, controlling electronic devices, power management, and audio/video signal processing. Discrete semiconductor devices generally contain one type of electrical component, e.g., light emitting diode (LED), small signal transistor, resistor, capacitor, inductor, diodes, rectifiers, thyristors, and power metal-oxide-semiconductor field-effect transistor (MOSFET). Integrated semiconductor devices typically contain hundreds to millions of electrical components. Examples of integrated semiconductor devices include microcontrollers, application specific integrated circuits (ASIC), power conversion, standard logic, amplifiers, clock management, memory, interface circuits, and other signal processing circuits.
A power conversion circuit converts an input voltage (AC or DC) to a second regulated direct current (DC) output voltage. The regulated DC output voltage is typically a different value than the input voltage. A power conversion circuit is used for any electronic device functioning with a DC operating voltage that derives its power from an alternating current (AC) source. For example, a television that plugs into an AC wall socket uses a power converter to convert the AC power to DC operating potential for the electronic components in the television. The power conversion circuit is also commonly used for charging the battery of a mobile device, e.g., cell phone and laptop computer.
During initial power-on when the AC or DC input voltage is applied, the components of the power conversion circuit undergo a number of transient states or conditions as the system transitions from zero output voltage to steady-state regulating operation. The transient states during power-on can be stressful on the electronic components of the power conversion circuit. A soft start circuit is commonly used in the power converter to restrict the range of transient states and reduce the stress on the components during power-on, i.e., the soft start circuit slowly brings the power converter from zero output voltage to a steady-state regulating mode.
The power converter can be implemented in a number of configurations. For example, a fixed frequency flyback, quasi-resonant flyback, and active clamp flyback are types of power converter circuits. These flyback power converters all benefit from a soft start circuit, although each topology has individualized requirements. One way to implement soft start is to slowly increase the inductor/transformer current limit from a small value (which restricts the range of component transient conditions) to larger values at a fixed operating frequency and variable pulse width to the switching power transistor until the final regulated DC output voltage is achieved. The slow increase in current limit produces a monotonic output voltage ramp. The ramped inductor/transformer current at fixed operating frequency and variable pulse width to the switching power transistor is sufficient to address many soft start applications. However, the active clamp flyback converter exhibits a number of pushing factors that must be addressed for optimal soft start protection. In particular, the active clamp flyback converter has several distinct start-up time periods that are susceptible to shoot-through current through stacked power transistors causing overheating if not properly controlled. Overheating during startup can degrade components and reduce the reliability over time, resulting in lower mean time between failure of the power converter.